Various cap liners and methods of producing cap liners are known in the patented prior art as evidenced by the patents to Dukess U.S. Pat. Nos. 4,107,247, 3,819,460, 3,595,419 and 3,976,217.
Such liners have been provided as a multilayer sandwich having one or more solid low density polyethylene outer layers and a flexible and resilient foamed inner or intermediate layer of a rubber-like material such as polyethylene, ethylene vinyl acetate, or the like. Cap liners of this type have been manufactured by way of simultaneous multiple extrusion using a combination dye for bonding the layers together.
An important feature of these cap liners is that the inner or intermediate layer expands outwardly beyond the outer layer or layers upon compression between the container and the cap, thereby abutting against the side walls of the cap to produce an effective seal. Such cap liners also have the advantages of being stress and crack resistant, bendable, compressible, and impervious to moisture, chemicals and acids when formed of appropriate materials.
Although known cap liners have proved to be economical and effective for sealing containers when not exposed to heat, such liners melt and/or weaken when used with caps on hot filled containers, thereby decreasing the effectiveness of the seal. Materials such as polypropylene are strong and heat resistant and thus would be useful in cap liners for hot filled containers. Up to the present time, however, it has not been possible to bond a polypropylene layer to another layer or layers formed of polyethylene in multilayer cap liners. Also, it has been difficult to foam polypropylene so that it could be used for the intermediate foamed layer in such cap liners. As a result of these problems, polypropylene has not been used in such cap liners.